Distributed feedback (DFB) semiconductor lasers are known. Such lasers comprise a feature that serves to suppress emission of radiation of all but one longitudinal spatial mode. In one well known type of DFB laser the feature is a "grating", e.g., a "corrugated" interface between two semiconductor layers. The grating is spaced from the active region of the laser and can be located either below or above the active region.
The degree of interaction of the radiation field in the laser cavity (of length L) with the grating is generally expressed in terms of the coupling constant .kappa., a parameter that depends on the spacing between the active region and the grating, and also depends on the "strength" of the grating. Maintenance of .kappa. within relatively narrow limits is an important manufacturing requirement for at least some grating DFB lasers, e.g., for such lasers that are intended for use in analog applications such as optical fiber analog CATV. See, for instance, U.S. Pat. Nos. 5,012,484 and 5,111,475, both incorporated herein by reference. In these lasers .kappa.L is desirably within relatively narrow limits of a predetermined value that maximizes linearity of the laser output as a function of drive current. For instance, the specifications for one such device call for the normalized variation of .kappa.L (i.e., .delta.(.kappa.L)/.kappa.L) to be at most about 13%. This is a requirement that is difficult to meet with at least some prior art grating DFB lasers, resulting in low manufacturing yield, and consequently, high cost of the lasers.
Among the reasons for the difficulty of close control of .kappa.L is the difficulty of close control in a manufacturing environment of the spacing between the active region of the laser and the grating. For instance, it is known that even a minor departure of the wafer temperature from the target temperature during layer growth can significantly affect the layer growth rate. Furthermore, applicable growth techniques (e.g., MOCVD) typically result in at least some layer thickness variation across the wafer.
In view of the commercial significance of grating DFB lasers with closely controlled value of .kappa.L, it would be desirable to have available such lasers that are less subject to .kappa.L variation than are corresponding prior art lasers. This application discloses such laser.